In gas turbine engines tip clearance between the compressor or turbines and the surrounding duct is kept as small as possible, to force as much air as possible in the gas path through the blades in order to maximize engine efficiency. Tip clearance is ideally optimized for an engine's hot running conditions, however, this can create “pinch” points, in which the difference in thermal expansion between the blades and the surrounding duct causes the blades to bind on the duct walls. Historically, tip clearance has been sized to prevent the occurrence of pinch points, however this is not optimal for hot (normal) running conditions. A well-known solution is so-called “active tip clearance control” in which cooling air is used to shrink the duct size during hot running conditions in order to minimize tip clearance during normal running conditions. The cooling air is turned off, for example during start-up and other flight stages in order to allow the duct size to expand enough to prevent pinch points. Numerous configurations of active tip clearance control systems are known. Conventionally, either the turbine support case which supports the turbine static shrouds, and/or the turbine static shrouds themselves are cooled. One perennial issue with active tip clearance control systems is where to take the cooling air from, and how to provide it to the area to be cooled.
Accordingly, there is a need to provide an improved active tip clearance control system for gas turbine engines.